1. Field of the Present Invention
The present invention generally relates to system converter devices for video signals, and more particularly, to a system converter device for converting video signals based on an interlaced scanning system to video signals based on an sequential scanning system. The present invention has particular applicability to a system converter device for converting video signals based on a MUSE system to video signals based on a NTSC system.
2. Description of the Background Art
In a high definition television system, 1,125 scanning lines are defined per frame, the interlace ratio is defined as 2:1, and the aspect ratio is defined as 16:9. One channel of satellite broadcasting has a bandwidth of 27 MHz. In satellite broadcasting, video signals based on a high definition television system are bandwidth compressed to 8.1 MHz and transmitted through one channel. This transmission system is called "Multiple Sub-Nyquist Sampling Encoding" (hereinafter referred to as MUSE).
This MUSE transmission system is known as a multi-subsampling transmission system utilizing offset subsampling between two fields or two frames. In the MUSE transmission system, line sequential time axis integration (TCI) is employed, where the red color difference signal R-Y and the blue color difference signal B-Y are time axis compressed to 1/4, and the compressed signals are time axis multiplexed during the horizontal blanking period of the luminance signal Y. That is to say, the compressed red color difference signals R-Y are multiplexed on odd number lines, while the compressed blue color difference signals B-Y are multiplexed on even number lines.
When a reproduced image from video signals based on a high definition television system such as the MUSE transmission system, is to be displayed by a television receiver under using a sequential scanning system with 525 scanning lines per field, and with an aspect ratio of 4:3, for example, the video signals of the high definition television system are first converted to video signals based on a NTSC system, whereupon these converted video signals are sequentially scan. FIG. 1 shows such an example of a conventional system converter device.
Referring to FIG. 1, a satellite broadcasting signal received by an antenna 31 is applied to a tuner 32. The MUSE signal provided from tuner 32 is applied to an A/D converter 33, where it is converted to a digital signal to be applied to a signal processing circuit 34.
In the signal processing circuit 34, 525 lines of scanning line data are produced in accordance with the data of every even number horizontal scanning line of the MUSE signal. Also in signal processing circuit 34, a portion of the luminance signal Y and the blue color difference signal B-Y are extracted from each scanning line signal to convert the aspect ratio for achieving the aspect ratio of 4:3 required in the NTSC system. The signal processing circuit 34 also stores the image data of the extracted luminance signal Y and the blue color difference signal B-Y in a memory (not shown) in response to a clock signal having a frequency relevant to the MUSE system, whereupon the stored data is read out in response to a clock signal having a frequency relevant to the NTSC system. This causes the horizontal period of each scanning line to be time axis expanded from 29.6 .mu.sec of the MUSE signal to 63.5 .mu.sec of the NTSC system.
The MUSE signal provided from A/D converter 33 is also applied to a signal processing circuit 36 via a delay element 35 having a delay time of 1 horizontal period (hereinafter referred to as "1H"). Accordingly, the timing of the scanning line signal having the red color difference signal R-Y is multiplexed to coincide with the timing of the scanning line signal having the blue color difference signal B-Y multiplexed. "1H" is equivalent to 1 horizontal period of the high definition television system, and the period of time is 29.6 .mu.sec.
The signal processing circuit 36 produces the scanning line data of 525 lines according to the data from every odd number scanning line of the MUSE signal. Also, in signal processing circuit 36, a portion of the luminance signal Y and the red color difference signal R-Y are extracted from each scanning line signal to convert the aspect ratio to an aspect ratio of 4:3 required in the NTSC system. The signal processing circuit 36 stores the image data of the extracted luminance signal Y and the red color difference signal R-Y in a memory not shown, where the stored data is read out in response to a clock signal having a frequency relevant to the NTSC system. This causes the horizontal period of each scanning line to have the time axis expanded to 29.6 .mu.sec of the MUSE signal from 63.5 .mu.sec of the NTSC system.
A coefficient switching-selecting adder circuit 37 receives the scanning line signal having a blue color difference signal B-Y multiplexed from signal processing circuit 34. Furthermore, this scanning line signal is applied to coefficient switching-selecting adder circuit 37 via a delay element 38 having a delay time of one horizontal period (1H'). "1H'" indicates one horizontal period of the NTSC system, in which the period of time is equivalent to 63.5 .mu.sec.
The MUSE signal provided from A/D converter 33 is also applied to a field discrimination circuit 39. The discrimination signal provided from field discrimination circuit 39 is applied to coefficient switching-selecting adder circuit 37 as a control signal. The coefficient switching-selecting adder circuit 37 adds the signal including the current signal multiplied by a coefficient of 1/4 and the signal including the 1H' delayed signal multiplied by a coefficient of 3/4 in response to the discrimination signal, to process odd number fields. To process even number fields, the signal including the current signal multiplied by a coefficient of 3/4 and the signal including the 1H' delayed signal multiplied by a coefficient of 1/4 are added.
Coefficient multiplier 40 multiplies by a coefficient of 1/2 with the scanning line signal provided from coefficient switching-selecting adder circuit 37 having a blue color difference signal B-Y multiplexed. Similarly, coefficient multiplier 41 multiplies by a coefficient of 1/2 with the scanning line signal provided from signal processing circuit 36 and having a red color difference signal R-Y multiplexed. The output signals from coefficient multipliers 40 and 41 are added by an adder 42 to produce a luminance signal Y.
It can be appreciated from the previous description that since coefficient switching-selecting adder circuit 37 switches the coefficients by which the current signal and the 1H' delayed signal are multiplied in each of the even number field and the odd number field, the luminance signal Y provided from adder 42 has an interlaced position between the mutual adjacent odd number field and even number field. The luminance signal Y provided from adder 42 is converted to an analog signal by a D/A converter 43 to be applied to a NTSC encoder 44.
The output signal of signal processing circuit 34 is also provided to a time axis expanding circuit 45, where the blue color difference signal B-Y having the time axis compressed to 1/4 the period of time is expanded. The expanded blue color difference signal B-Y is provided to an intra-field interpolation circuit 46, where weighted mean processing between the two scanning line signals is performed. The output signal of intra-field interpolation circuit 46 is converted to an analog signal by a D/A converter 47 to be provided to NTSC encoder 44.
The output signal of signal processing circuit 36 is provided to a time axis expanding circuit 48, where the red color difference signal R-Y having the time axis compressed to 1/4 the period of time is expanded. The expanded red color difference signal R-Y is provided to an intra-field interpolation circuit 49, where weighted mean processing between the two scanning line signals is performed. The output signal of intra-field interpolation circuit 49 is converted to an analog signal by a D/A converter 50 to be provided to NTSC encoder 44.
The MUSE signal provided from A/D converter 33 is also applied to a synchronizing signal generating circuit 51. The synchronizing signal generating circuit 51 is responsive to a synchronizing signal included in the MUSE signal to generate a synchronizing signal according to the NTSC system. This synchronizing signal of the NTSC system is applied to encoder 44. The encoder 44 adds the synchronizing signal to the luminance signal Y, as well as, produces a carrier chrominance signal C by quadrature two-phase modulation of the color difference signals R-Y and B-Y. By adding the luminance signal Y and the chrominance signal C, a video signal SV for the NTSC system is generated.
The video signal SV generated from encoder 44 is converted to a digital signal by an A/D converter 52 to be applied to a Y/C separation circuit (luminance signal/color signal separation circuit) 53. The separated luminance signal Y from Y/C separation circuit 53 is applied to a scanning line interpolation circuit 54. The separated chrominance signal C from Y/C separation circuit 53 is provided to a color demodulation circuit 55. The color difference signals R-Y and B-Y provided from color demodulation circuit 55 are applied to scanning line interpolation circuit 54. The scanning line interpolation circuit 54 generates interpolation scanning line signals in accordance with the main scanning line signal of each luminance signal Y and color difference signals R-Y and B-Y, respectively. The main scanning line signals and the interpolation scanning line signals of the luminance signal Y, the color difference signals R-Y and B-Y provided from scanning line interpolation circuit 54 are applied to a double speed conversion circuit 56, where double speed conversion process is carried out. That is to say, each interpolation scanning line signal is inserted between the main scanning line signals. As a result, a luminance signal Y', color difference signals R'-Y' and B'-Y' of the sequential scanning line system having 1 horizontal period compressed to H'/2 (525 lines/field) are generated.
The video signal SV provided from encoder 44 is applied to a synchronizing signal generating circuit 57. The synchronizing signal generating circuit 57 generates a synchronizing signal according to the sequential scanning line system in response to a synchronizing signal based on the NTSC system. The synchronizing signal of the sequential scanning system is provided to an adder 58. The double speed conversion circuit 56 applies the luminance signal Y' to a D/A converter 59, where the luminance signal Y' is converted to an analog signal. The adder 58 adds the synchronizing signal to the converted luminance signal Y'.
Each color difference signals R'-Y' and B'-Y' provided from double speed conversion circuit 56 are converted, to analog signals by D/A converters 61 and 62 to be applied to modulation circuits 63 and 64, respectively. At each modulation circuit 63 and 64, two subcarriers having a phase difference of 90.degree. to each other are balance modulated in response to the color difference signals R'-Y' and B'-Y'. The output signals from modulation circuits 63 and 64 are added at adder 65, where a carrier chrominance signal C' is generated.
A monitor receiver 60 receives a luminance signal Y'' having a synchronizing signal added from adder 58, and a chrominance signal C' provided from an adder 65. Consequently, an image according to the sequential scanning system of 525 scanning lines/field (non-interlaced system) is displayed on the display screen of monitor receiver 60.
From the above description of the circuit shown in FIG. 1, the 1,125 horizontal scanning lines of the MUSE signal is first reduced to 525 lines and then increased to 1,050 lines by the double speed conversion process. Such a process causes degradation in the quality of the displayed image, in addition to increasing the size and complexity of the circuit configuration. Furthermore, since MUSE signals are based on line sequential TCI, the separated luminance signal Y and the color difference signals R-Y and B-Y are mixed to produce a video signal for the NTSC system. It is pointed out that the process of sequential scanning conversion by separating these mixed signals again causes degradation in the quality of the picture.